1. Field of the Invention
This invention relates broadly to orthopedic devices. More particularly, this invention relates to systems for locking bone screws relative to bone plates.
2. State of the Art
For various fractures of bones of the body, plating is a well known technique to impart the stabilization desirable for proper healing. In plating, a rigid, typically metal plate is placed on the outer surface of the bone across the fracture, and screws are secured through the plate and into the bone on either side of the fracture in a manner which permits the rigid plate to offer support to the bone during healing.
The screws include threads along a shaft adapted to engage cortical bone. Most commonly, the head portion of the screw is generally a standard screw head which provides a compressive force about a corresponding round screw hole of the plate as the screw is thread into the bone, thereby causing compression of the plate against the bone.
U.S. Pat. No. Re. 28,841 to Allgower describes a plate that is used with generally standard screws having heads with a convex undersurface. The plate includes oblong screw holes which each define at one end an upper ramped portion and a generally smaller radius of curvature about the ramped portion. In use, a hole is drilled into the bone through the screw hole adjacent the ramp and a screw is inserted through the screw hole adjacent the ramp into the drilled hole and rotated until the head of the screw contacts the ramp. Upon such engagement, there is displacement of the bone plate in a direction to move the ramped portion away from the screw and the cause the plate to apply pressure to maintain the bone parts together about the fracture in tight engagement.
More recently, particularly at the metaphysis of long bones though not limited thereto, there have been desirable results with screws having exteriorly threaded heads which threadably engage threads in the plate to lock the screws relative to the plate and thereby limit compression of the plate relative to the bone. However, such systems do not provide the necessary control of compression between the plate and bone. Control over compressive forces is lost as soon as the threads of the head of the screw lock relative to the plate. Therefore, such a system provides sub-optimal stability.
Certain plates sold by Synthes are designed with a hole called a COMBI-HOLE™. The COMBI-HOLE™ is an elongated screw hole including two joined circular sections, each extending through approximately 250°. One of the circular sections is threaded and thus adapted to received a screw with a threaded head at a fixed angle. When used as such, the system has the same lack of control over compression as discussed above.